Constraints: add checks to specially handle the custom space target.

- The custom space target never needs B-Bone data (used by depsgraph).
- When drawing the relationship lines use the space matrix directly.
- Don't use the custom target to control the target space type dropdown.

Differential Revision: https://developer.blender.org/D9732

source/blender/blenkernel/intern/constraint.c

@@ -5844,9 +5844,12 @@ static bConstraint *add_new_constraint(Object *ob,
   return con;
 }
 
-bool BKE_constraint_target_uses_bbone(struct bConstraint *con,
-                                      struct bConstraintTarget *UNUSED(ct))
+bool BKE_constraint_target_uses_bbone(struct bConstraint *con, struct bConstraintTarget *ct)
 {
+  if (ct->flag & CONSTRAINT_TAR_CUSTOM_SPACE) {
+    return false;
+  }
+
   return (con->flag & CONSTRAINT_BBONE_SHAPE) || (con->type == CONSTRAINT_TYPE_ARMATURE);
 }
 

source/blender/draw/engines/overlay/overlay_extra.c

@@ -1319,7 +1319,10 @@ static void OVERLAY_relationship_lines(OVERLAY_ExtraCallBuffers *cb,
 
           for (ct = targets.first; ct; ct = ct->next) {
             /* calculate target's matrix */
-            if (cti->get_target_matrix) {
+            if (ct->flag & CONSTRAINT_TAR_CUSTOM_SPACE) {
+              copy_m4_m4(ct->matrix, cob->space_obj_world_matrix);
+            }
+            else if (cti->get_target_matrix) {
               cti->get_target_matrix(depsgraph, curcon, cob, ct, DEG_get_ctime(depsgraph));
             }
             else {

source/blender/makesrna/intern/rna_constraint.c

@@ -603,7 +603,7 @@ static const EnumPropertyItem *rna_Constraint_target_space_itemf(bContext *UNUSE
 
   if (BKE_constraint_targets_get(con, &targets)) {
     for (ct = targets.first; ct; ct = ct->next) {
-      if (ct->tar && ct->tar->type == OB_ARMATURE) {
+      if (ct->tar && ct->tar->type == OB_ARMATURE && !(ct->flag & CONSTRAINT_TAR_CUSTOM_SPACE)) {
         break;
       }
     }